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> Insects, especially rapid flie
Hi Steve & All, Oct 10, 2010
Thanks for the additional comments Steve. In my earlier post I left out
one aspect from the small is beautiful context by oversight;
i.e. being specialized in many (most ?) cases for small or narrow food
sources (nectar, pollen, spores, fungal hyphae, cambium/phloem, grass, fine
roots, leaf mining, under bark--- there would be distinct (perhaps lethal)
disadvantages to much larger size. A 5 cm long beetle that specialized in
myxomycete spores is one of those creatures that didn't catch the ark.
If many must be small, to exploit their food resources effectively,
then the choices made in eons past in the small size direction may constrain
those who could be larger (e.g. leaf feeders) to also remain relatively
small.
One interesting size effect is the minimum size of fire and solid fuel
that will burn in a practical way and which in turn requires arms that are
long and strong enought to feed fuel onto the fire. I am not sure what this
minimum size is (75 cm ?) but if humans had been too small to tame fire (the
one quality that sets us apart from, and gives us dominion over, other
animals) then we might be endangered or extinct.
. Yt, Dave Webster, Kentville
----- Original Message -----
From: "Steve Shaw" <srshaw@DAL.CA>
To: <naturens@chebucto.ns.ca>
Sent: Friday, October 08, 2010 1:38 PM
Subject: Re: size considerations: was Re: [NatureNS] Wasp question (long)
> Hi Dave, Paul & others,
> The many complicated formulations of how diffusion occurs are modelled
> mainly on the formally similar process of heat conduction along rods
> and through slabs etc, where industrial application needs have led to
> more widespread analysis (eg. classic book by Carslaw & Jaeger). The
> rate of supply will indeed vary according to whether the oxygen is
> removed instantaneously at the far end, or allowed to build up, in the
> long term though not initially. In terms of the evolutionary pressure
> to branch further, I don't think this argument is valid. The reason is
> that the rate of movement of diffusant along a channel or tube in the
> direction of its length is independent of the width of the channel
> until you get down to molecular dimensions (wall effects). If you
> imagine that all the branched-off channels have the same diameter as
> the original one, that would be similar to instantaneously widening
> the pathway (increases cross-sectional area), which would have a
> serious effect (the first part of the pathway would be limiting).
> However, this is not how tracheae branch and sub-branch again, but
> rather like the branches of a tree, with the deeper branches getting
> finer and finer until they enter the muscles; I'm not sure if the
> actual aggregate cross-section remains exactly constant, but it
> probably approaches that. Most of the tracheal system has hydrophobic
> walls which is partly why they don't fill with water when an insect is
> immersed, but the tips of the final branches, tracheoles, are
> hydrophylic and contain fluid where gas exchange occurs. When the
> muscles are active, I think it was Wigglesworth who showed that this
> fluid withdraws deeper into the muscle so the interface moves closer
> to site of oxygen demand.
>
> I cut a few corners in the earlier e-mail: some insects in some parts
> of their bodies actively pump the tracheal system locally by
> contracting nearby muscles, so this presumably aids movement of gases
> along, beyond the constraints of simple diffusion. The most obvious
> example though adapted for a different use would be that of the
> Madagascar hissing cockroach, which when alarmed expels air rapidly
> from the enlarged tracheae connected to one pair of spiracles. And in
> addition to the series of segmental hearts, blood is also circulated
> up long appendages like the antennae by little pumping stations
> ('accessory hearts') at their bases.
>
> I assumed that the old story about sauropods in swamps was still
> correct, but Paul sets this on its head. I agree that the likely
> reason why the large carboniferous dragonflies could function is that
> their bodies while very long were not all that wide, from memory of a
> diorama somewhere illustrating this. The tracheae basically run across
> the body segments from the sides, but in addition there are
> longitudinal tubes that connect up the tracheae between segments in
> most insects as well.
> Interesting, in light of the earlier recommendation of Vincent
> Dethier's beautifully written books (the comment on flight in
> particular though I'd forgotten that he treated that), it has emerged
> more recently that small insects don't fly by the same mechanism as
> larger birds. From Michael Dickinson's work, air viscosity is really
> important at small size, and small insects generate lift quite
> differently by having air vortices roll off the wings. This raises
> the interesting question of whether those large ancient dragonflies
> would have used more conventional lift mechanisms as they were as
> large or larger than many modern birds. I haven't followed this
> recently but assume that people will now be looking at hummingbirds.
>
> Dave, there's a lot of info about heart rate in different insect
> species (tables in a book by J. C. Jones for instance), and I guess
> that circulation rate goes up as size does, but am not sure if this is
> well documented without further digging. You may know already that
> fancy insect flight muscle doesn't work well when cold, which is
> another likely reason for sunning, to increase its temperature. It is
> known that this happens during warm-up when some moths vibrate their
> wings for several minutes -- the temperature in flight muscle goes up
> by several degrees, allowing them finally to take off.
> Steve, Halifax
>
>
>
> Quoting David & Alison Webster <dwebster@glinx.com>:
>> ----- Original Message ----- From: "Steve Shaw" <srshaw@DAL.CA>
>> To: <naturens@chebucto.ns.ca>
>> Sent: Sunday, October 03, 2010 1:38 PM
>> Subject: Re: [NatureNS] Wasp question (long)
>>
>>
>>> Hi Paul, Derek, Andy, Dave and all,
>> So if you
>>> double the width of the insect, you double the length of the tube
>>> pathway (*2), but the rate of movement of the gases O2 and CO2 will
>>> drop to one quarter of what it had been before, at the tissue end
>>> (1/(2 squared) = 1/4). This is believed to be one of the main factors
>>> that limits the ultimate size of insects, such that at large size they
>>> simply cannot supply O2 to the tissues fast enough by passive
>>> diffusion. The most energetically expensive tissue known is insect
>>> flight muscle, to give an idea of why this might be important.
>>
>> Hi Steve & All, Oct 7, 2010
>> It is correct that if delivery of a gas by diffusion to the end
>> of a tube (of uniform diameter) is J then delivery to the end of
>> same diameter tube will be J/4, if tube length is doubled.
>>
>> And-- the above applies (I think) whether removal of gas from the
>> sink end of the tube is at a very rapid rate relative to the rate of
>> diffusion or at a slow rate; if removal is rapid then the
>> concentration gradient will be steep and if slow then shallow.
>>
>> But if this dimensional effect were relevant, why over all these
>> years, would not evolutionary pressures have developed sufficient
>> branching in larger insects to compensate for this effect ?
>>
>> Unless I have missed something, the answer is quite simple. Most
>> insects have evolved with flight somewhere in the background and if
>> you are going to fly then small is beautiful. For example, fairly
>> large Mayflies can almost float in thermals generated by a canoe or
>> wharf at calm twilight. And smaller insects can ride the tiny
>> thermals generated by shrubs or other localized heat sources.
>>
>> Also, unless transfer of O2 from the trachea to tissue fluids,
>> followed by flow away from the trachea, is very efficient, and it is
>> difficult to see how it could be*, then the really steep
>> concentration gradient would be at the gas/liquid interface.
>>
>> *Based on O2 solubility in water at 20oC; 20% O2 in air would be in
>> equilibrium with about 0.6% O2 by volume in water.
>>
>> Insects, especially rapid fliers, can often be seen sunning
>> themselves. And it has seemed reasonable to me (from first
>> principles) that this provides time to digest the last meal and
>> deliver the products of digestion to the points of use; this
>> delivery must be less satisfactory as size increases unless offset
>> by better circulation
>>
>> Just shooting from the hip,
>> Dave Webster, Kentville
>
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